The invention relates to a metal halide lamp provided with a discharge vessel with a ceramic wall which encloses a discharge space containing an ionizable filling which comprises besides Hg a molar quantity of halides of Na, Tl and at least one of the elements Dy and Ho.
A lamp of the kind mentioned in the opening paragraph is known from EP-A-0 215 524. The known lamp, which combines a high luminous efficacy with excellent color properties (among them general color rendering index R.sub.a .gtoreq.80 and color temperature T.sub.c between 2600 and 4000 K) is highly suitable for use as a light source for inter alia interior lighting.
In this lamp, the recognition is utilized that a good color rendering is possible when Na halide is used as a filling ingredient of a lamp, and that a strong widening and inversion of the Na emission in the Na-D lines occurs during lamp operation. This requires a high temperature of the coldest spot T.sub.cs in the discharge vessel of, for example, 1170 K (900.degree. C.). When the Na-D lines are inverted and widened, they assume the shape of an emission band in the spectrum with two maxima at a mutual distance .DELTA..lambda..
The requirement that T.sub.cs should have a high value excludes under practical circumstances the use of quartz or quartz glass for the discharge vessel wall, and necessitates the use of a ceramic material for the discharge vessel wall.
A ceramic wall in the present description is understood to be a wall made from metal oxide, such as, for example, sapphire or densely sintered polycrystalline Al.sub.2 O.sub.3, as well as from metal nitride, for example AlN.
The known lamp has a good color rendering and also a comparatively wide range for the color temperature.
In general, the known lamp is operated on an AC voltage supply source with a frequency of no more than 120 Hz. The discharge will be extinguished and subsequently be re-ignited in the lamp, once this has been ignited, upon each polarity change in the supply voltage. This re-ignition takes place at a voltage level, called re-ignition voltage hereinafter, which is higher than the stable arc voltage of the lamp. The ratio of the re-ignition voltage to the arc voltage is called crest factor. The crest factor assumes a comparatively high value in particular when the lamp is operated on a sinusoidal signal. The crest factor usually increases in value during lamp life. The lamp will not re-ignite anymore and remain off when the crest factor assumes a too high value. The required quantity of metal halide is found to lead to very high initial values for the crest factor and to a fast rise thereof through lamp life when a lamp having a color temperature T.sub.c in the range between 3900 K and 4500 K is realized. This adversely affects lamp life.